The present invention relates generally to the reduction of nitrate in aqueous solutions and, more particularly, to the electrochemical reduction of nitrates in the presence of amides.
Nitrate-containing solutions are used in weapons research and production. As a result, nitrate-containing wastes are generated. The steel, mining and chemical industries, and farming are also major generators of nitrate wastes. Such wastes contaminate natural waters if released into the environment; the strong oxidizing power of nitrates causes excessive biological activity, growth of the vegetation, and precipitation of organic residue to the bottoms of streams, rivers, lakes, and oceans. As a result of precipitation of organic residue from nitrate waste streams, water reservoirs become shallow and eventually are converted to marshes and decay. Nitrate contamination in drinking water can cause methemoglobinemia, which is especially detrimental to infants and nursing mothers.
Several methods for treating nitrates in waste streams have been developed which convert the nitrates to less environmentally hazardous substances, but no simple, economical process for the destruction of nitrates in aqueous solutions is known. Nitrates can be separated by physiochemical means such as reverse osmosis, ion exchange, electrodialysis, and evaporation, but these methods do not destroy the nitrate.
Although nitrates can be reduced to nitrogen gas and ammonia by biological procedures, this process is slow, difficult to control and results in an organic residue.
Supercritical water or wet air oxidation methods have also been applied to the destruction of nitrates. However, these methods require high temperatures and pressures, the inconsistent contents or concentrations of waste streams are problematic, and the products of nitrate reduction with supercritical fluids may contain undesirable NOx components.
In U.S. Pat. No. 6,030,520, for xe2x80x9cNitrate Reductionxe2x80x9d which issued to Jacek J. Dziewinski and Stanislaw Marczak on Feb. 29, 2000, a method for solving this problem is described. Aqueous waste streams containing nitrates are first contacted with a metal such as cadmium or zinc in an amount sufficient to quantitatively reduce the nitrates to nitrites (NO3xe2x88x92to NO231 ).
Both the nitrites and the metal cations are released into the solution or slurry that contained the nitrates. The resulting aqueous nitrite solution is then contacted with an amide reagent to reduce the nitrites to nitrogen and carbon dioxide or acid anions. The reduction of the nitrites to nitrogen may be carried out simultaneously with further nitrate reduction to nitrites or subsequent to the nitrate reduction. Reduction may be performed in the same vessel as the nitrate reduction or the solution may be pumped to another vessel. Amides which are useful in the practice of the invention are those which produce environmentally benign products during hydrolysis; for example, urea, sulfamic acid, formamide, acetamide and mixtures thereof. Urea and sulfamic acid reduce nitrites to nitrogen and carbon dioxide or sulfate anions, respectively. Use of too little amide results in incomplete conversion of the nitrites to nitrogen, while too much amide causes unnecessary reagent use and its residual presence in the treated solution. The pH of both the nitrate to nitrite reaction and the nitrite to nitrogen reaction is adjusted as necessary to obtain a weakly acidic reaction environment. When the aqueous solution or slurry is subjected to a direct current between an anode and cathode submersed therein, the cadmium, zinc or other metal will plate out on the cathode and can be recovered therefrom. Metal recovery by electroplating may be carried out in the same vessel as the nitrate reduction and may be carried out simultaneously with the nitrite to nitrogen reduction reaction, simultaneously with the nitrate to nitrite reduction reaction after the reduction step is completed, or as a final step of the overall process.
In U.S. Pat. No. 5,871,620 for xe2x80x9cProcess And Device for Reducing The Nitrate Content Of Waterxe2x80x9d which issued to Helmar Haug et al. on Feb. 16, 1999, nitrate is reduced to nitrogen in two stages. An ultraviolet photochemical reaction reduces the nitrate to nitrite followed by a reduction of the nitrite to nitrogen using amidosulfuric acid.
Direct electrochemical reduction of aqueous inorganic oxynitrogen species to nitrogen or ammonia is described in U.S. Pat. No. 5,376,240 for xe2x80x9cProcess For The Removal Of Oxynitrogen Species For Aqueous Solutionsxe2x80x9d, which issued to Jerry J. Kaczur et al. on Dec. 27, 1994. The aqueous solution is fed into the catholyte compartment of an electrochemical reduction cell using a high surface area cathode separated form an anolyte compartment to electrochemically reduce substantially all of the oxynitrogen species to nitrogen or ammonia and produce a purified water product.
S. Glastone and A. Hickling on page 223 of xe2x80x9cElectrolytic Oxidation and Reduction: Inorganic and Organicxe2x80x9d, D. Van Nostrand Company (New York 1936) state that during electrolysis at a copper cathode HNO3 is generally reduced to ammonia, but the action of the acid on this metal yields nitric oxide as the chief product. It is further stated that this is the result of the presence of cupric ions during dissolution, and the absence of cupric ions during electrolysis. When Cu++ ions are effectively removed during acid dissolution (quickly flowed away, precipitated and filtered) ammonia is principally formed. Reversibly, when copper sulfate is present during the electrolysis, nitric oxide easily evolves.
This reference also quotes Wilikinson, Trans. Amer. Plectrochem. Soc., 13, 309 (1908) as stating that electrolytic reduction of HNO3 at a mercury cathode yields a large proportion of hydroxylamine, but if mercurous sulfate is added then nitric oxide can be obtained with 70% current efficiency.
At alkaline conditions a mixture of ammonia and nitrite were reported as products of nitrate electrolysis on copper, silver, iron and lead (See, e.g., Miller and Weber, Z. Elektrochem. 9, 955 (1903). Ammonia and very little nitrite resulted during nitrate electrolysis on platinized platinum (Muller and Spitzer, Ber. 38, 1190 (1905)). Saturated pH neutral solutions of sodium nitrate may be reduced to nitrite at an amalgamated copper cathode according to W. J. Muller, Z. Elektrochem. 9, 978 (1903)). Nitric oxide has been detected as a product of electrolysis of concentrated nitric acid with a platinum electrode (Glastone and Hickling, supra). In xe2x80x9cElectrochemical Reduction Of Nitrates And Nitrites In Alkaline Nuclear Waste Solutions, by J. D. Genders et al., J. AppI. Electrochem. 26, 1 (1996), alkaline solutions of sodium nitrate and nitrite are electrochemically reduced to nitrogen, ammonia or nitrous oxide in a divided electrochemical flow cell using a lead cathode, a cation exchange membrane, and oxygen-evolving DSA or platinum clad niobium anode.
Nitrous oxide has been obtained by the reduction of xe2x89xa71 M HNO3 in sulfuric acid at a mercury cathode (See, e.g., Tscherbakov and Libina, Z. Elektrochem. 35, 70 (1929)). It may be considered as a dehydration product of hyponitrous acid, which is suspected to be a transient product of nitrate reduction. At concentrations lower than 1 M hydroxylamine it is the principal product.
Nitrogen was obtained as the principal reduction product with dilute HNO3 (sp. Gr. 1.05) at very low current densities (1 to 5xc2x710xe2x88x925 A/cm2) at lead and silver cathodes (See, e.g., Freer and Higley, Amer. Chem. J. 21, 389 (1899)). According to Glastone and Hickling, supra, the fact that N2 is obtained at very low current densities is evidence that it results from a side reaction and not from direct electrolysis. They propose the following reactions:
2HNO+2NH2OH=2N2+4H2O, or
HNO2+NH3=NH4NO2=N2+2H2O.
The formation of ammonia at a platinum cathode was observed by some researchers but not by others. It has been reported by Ihle in Z. Physikal. Chem. 19, 572 (1896) that unless a certain current density, which depends on the concentration of the acid is exceeded, no ammonia is obtained. This critical current density increases with HNO3 concentration. The minimum current density undoubtedly varies with the electrode material, for at a suitable copper cathode, or in presence of a copper salt, nitric acid can be readily and completely reduced to ammonia even at quite low current densities.
Although it appears that HNO2 plays a major role in reduction of nitrates, there is considerable controversy on the mechanism of the reduction of nitrate to nitrous acid. See, e.g., Schmidt G., Ber. Bunsenges. 63, 667 (1959); G. Schmidt, Ber. Bunsenges. 65, 531 (1961); G. Schmidt and G. Krichel, Ber. Bunsenges. 68, 667 (1964); and G. Schmidt and M. A. Lobeck, Ber. Bunsenges. 73, 667 (1969)).
Thus, there remains a present need to efficiently and inexpensively destroy nitrates in solutions thereof using robust and simple apparatus without generating environmentally undesirable substances.
Accordingly, it is an object of the present invention to provide a method for converting nitrate wastes at moderate temperatures and pressures to more environmentally benign substances.
It is another object of this invention to provide a method of reduction of nitrates to nitrogen gas.
Additional objects, advantages and novel features of the invention will be set forth, in part, in the description that follows, and, in part, will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
To achieve the foregoing and other objects of the present invention, and in accordance with its purposes, as embodied and broadly described herein, the method for reducing nitrate in aqueous solutions thereof to nitrogen (N2) includes the step of electrochemically reducing the nitrate in the presence of at least one amide, whereby nitrogen is formed.
It is preferred that the amide is selected from the group consisting of urea, sulfamic acid, formamide, and acetamide.
In another aspect of the present invention in accordance with its objects and purposes, the method for reducing nitrate in aqueous solutions thereof to nitrogen (N2) includes the steps of electrolytically reducing the nitrate thereby forming a solution containing aqueous nitrites, and mixing said solution of nitrites sufficiently rapidly with at least one amide such that nitrogen (N2) is the principal product therefrom.
Preferably, the amide is selected from the group consisting of urea, sulfamic acid, formamide, and acetamide.
Benefits and advantages of the invention include an efficient, economical procedure for reducing the quantity of aqueous nitrates without generating environmentally harmful products, which can be performed using a simple apparatus.